Discharge tube



` Nov.. l2, 1935. P KAPTEYN 2,021,102

DISCHARGE TUBE Filed Feb. 2, '1932 m @13.5. QM

Patented Nov. 12, 1935 DISCHARGE TUBE Paul Kaptein, Berlin, Germany Application February 2, 1932, Serial No. 590,459 In Germany February 16, 1931 5 Claims.

In the case of discharge tubes it is important, for the purpose of obtaining maximum amplification per stage (aperiodic high frequency amplification) or the greatest possible independence of frequency (preferably in the case of low frequency amplification) to employ tubes having the smallest possible capacities between the single electrodes, which do not enter into the oscillation circuit capacity of oscillatory circuits, but are situated in parallel with chokes or resistances. The accomplishment of small capacities is successful to a very considerable degree by the use of screening grid tubes, as in the case of tubes of this nature the reaction of the anode causing in grid circuits the occurrence of the apparent capacity, increased in relation to the static grid anode capacity to the extent of the amplification factor, is capable of being reduced to an ineffective fraction of its normal Value.

In itself, however, the inclusion of auxiliary grids is accompanied by the disadvantage that the total of the static capacities is increased. For this reason, generally speaking, the use of screening grid tubes has not heretofore been taken into consideration in the case of aperiodic high frequency amplifiers.

Now the present invention discloses the manner in which amplifier tubes require to be constructed which fulfil to a very extensive degree the above requirements. The invention is based on the recognition that the total of the detrimental capacities of each anode circuit, and accordingly-so far as the tube itself is concerned--the total of the electrode capacities effective `between the anode and the cathode, requires to be kept small.

If now two detrimental capacities are formed by three electrodes, which, for example, are coaxial, whereby the one capacity is naturally formed by the outer and the middle electrode, and the second by the middle and the inner elecrode, a certain dimensicning of the middle electrode will exist with given inner and outer electrodes, which dimensioning reduces the total of these two capacities to a minimum. This minimum results, for example, in the case of cylindrical electrodes, of which the outer may possess the radius 111, the middle the radius rm and the inner electrode the radius r1, as condition for the lowest total capacity:

Im 1/ I'a'ti' This equation is a result out of the following mathematical thinking: It is where l states the length of the condenser. The capacity between anode and controlling grid is very small as a result of the screening effect of (Cl. Z50-27.5)

the screening grid and can therefore be neglected without noticeable mistakes.

For

Q drm this capacity is going to be a minimum, therefore is:

Fig. 4 is a diagrammatic showing of the two tubes shown in Fig. 3.

In Fig. 1 there is shown a possible form of embodiment of a resistance-coupled high frequency amplifier. There are employed here screening grid tubes havingthe screening grids SG1 and SGz, the anodes A1 and A2, the control grids G1 and G2, and the cathodes K1 and K2. The potential to be amplified is applied, for example, by means of an oscillatory circuit I, to the grid G1 of the first stage, so that the grid cathode capacity of the first stage enters into the oscillatory circuit. Now the screen grid SG1 is preferably disposed as far away as possible from the anode A1, in order to make the detrimental capacity between these two electrodes as small as possible. A relatively large capacity between the screening grid SG1 and the grid G1 does not cause disturbance, because this capacity also enters into the tuning of the oscillatory circuit l.

In Fig. 1 there is furthermore designated by 2 the anode resistance of the first stage, by 3 the grid resistance leak of the second stage, and by 4 the connecting coupling condenser.

The second stage, the output of which is connected from the anode A2, for example, over a tuned transformer 5, possesses two detrimental capacities, viz., between the cathode K2 and the grid G2 and between the grid G2 and the screening grid SG2, while the screening grid anode capacity enters into the tuning of the output tent towards the anode. As shownV by the invenf tion, assuming cylindrical electrodes are employed, it is then preferably, with a radius sg of the screening grid and a radius rt of the cathode, to so select the radius of the grid rg that rg is equal to To the tuned transformer there is connected either an additional high frequency amplifier, a rectifier or another apparatus, which is to be controlled bythe higlrfrequency` oscillations, for example, a television receiving apparatus or the like.

In Fig. 2 the two systems are illustrated diagrammatically in cross-section. The designations are the same as in Fig. l.

In the case of limited conditions of space, and with very high frequencies, it is frequently impossible to m'ake the detrimental capacities so small that, even when employing the screening grid, a sufficient degree 'of amplification is capable of being obtained. In these cases it is desirable to compensate the small extent of the degree of amplification, necessitated by the impossibility of obtaining a sufficiently large anode resistance, by an appreciable slope. This is-accomplished in convenient manner by the use of a space` charge grid. An arrangement of this kind is illustrated in a possible form of embodiment in Figs. 3 and l. In these figures the same designations have been employed as in Figs. l and 2. There is merely .the addition of the space charge grids-HG1 and RG2. The same considerations which were set forth in connection with the above examples are capable ofv being applied also in this case. In the iirst system there is a single detrimentalcapacity between the anode A1 and the screening grid SG1, so that the same should be selected as small as possible, while the Vcapacities between the grid Gi and the Screening grid SG1 between the grid G1 and the Yspace charge grid RG1,V and between the space charge grid RG1 and the cathode K1 Yare all without detrimental effect and, with the exception of one, enter into the tuning of the oscillatory circuit l. Inv the second system the capacities Vwithout detrimental effect are constituted by the Vcapacity between the screening grid SG2 and the anode A2 and between the space charge grid HG2 and the cathode K2. The remaining two capacities between SG2 and G2 and between G2 and RG2 are detrimental, and accordingly require to be maintained as small as possible. There is accordingly employed pursuant to the invention a screening grid SG2 as large as possible and a space charge grid RG2 as small aspo'ssible, and the intermediate gridG2 is dimensioni-:d in accordance with the equation set forth above.

It is not essential that the 'condition set forth above in respect of the lowest total capacity is adhered to perfectly strictly. The arrangements set forth in the above result in an amplification not obtained heretofore even in the case of certain variation, also in the case of frequencies, in connection with which an `effective amplification was hitherto impossible by resistance amplication. In the forms of embodiment set forth above the capacity of two electrodes, which'are separated by a third electrode, has not been taken into account, since, more particularly in the case of the narrow grids preferably employed, this capacity as compared with the capacities of electrodes which are not separated by additional electrodes is so small as to be capable of being ignored.

As already stated above, the invention is important, not only as regards aperiodic high frequency amplier for particularly high frequencies, but also in the case of high and low freduency amplifiers, in which the transmission of greatly varying frequencies is to be ensured in a'manner as even as possible, such, for example, as necessary in the case of television receiving apparatus and the like.

The amplifier systems illustrated in Figs. 1 4, including the coupling elements employed for connecting purposes, may convenientlybe arranged in a common vacuum space in the fornr of a multiple tube. A

As shown by more recent experiments, it is particularly desirable to make the grid cathode capacity vof theV rst stage larger than in the second stage. The reason for this is based on the facts following: It is possible without difficulty in the case of the second stage, which operates with tuned anode circuit, i. e., with a relatively high anode resistance, to obtain a very high degree of amplication, which 'may be extended so far that the system cannot be maintainedin a non-oscillating condition.

The Vproblem concerned accordingly consists in obtaining the greatest possible total famplication whilst maintaining freedom from oscillation. Since now the oscillation tendency of the second stage depends greatly on the loading of its grid and anode circuit, while on the other hand the amplification of the first tube is greatly dependent on non-capacity of its anode circuit and the following grid circuit, it is desirable to increase the amplification of the rst system as far as possible by lowering the effective "grid cathode capacity of the second stage, i. e., at the expense of amplication on the part of the second stage.

The amplication of .the secondV the greater grid cathode spacing necessary for lowering the capacity decreases the slope. The small grid cathode spacing of the rst stage then required acts only a favourable manner, as the slope, and accordingly the amplification, of this stageis particularly great. The high capacity which is caused by reason of the small spacing hasno detrimental effect insofar as the same enters into the tuning capacity ,of the input oscillatory circuit.

Particular advantage is furthermore obtained by selecting the diameter of the cathode to be as large as possible, in order to obtain a large emissive surface. Since in this manner the capacity betweenV grid and cathode is naturally considerably increased with the-same grid and cathode spacing, merely the first system of the amplifier is, in accordance with the invention, furnished with a cathode of relatively large diameter, while for the second system there is employed a cathode of relatively small diameter.

The invention is described more fully with the assistance of the drawing 5. In this l is an oscillatory circuit, which consists of a coil l and a tuning condenser 8, and to which there is supplied the control energy for example by an aerial I2 through the medium of a coil i3. This oscillatory circuit is situated at the grid G1 and the cathode K1 of the first stage of a two-stage amplier, to the anode A1 of which there is conducted the anode potential En via the anode resistance 2. The anode is connected through the medium of a condenser with the control grid G2 of the second stage, to which there is conducted the grid potential Eg via the resistance 3. The cathode of this second system is designated K2. Both cathodes K1 and K2 are heated indirectly by the filaments 9 and it, which may be furnished with heating energy from an A. C. main through a transformer M. The anode A2 of the second stage is connected with the anode potential source through the medium of an oscillatory structure consisting of the coils and 6, and the tuning condenser ifi. The oscillatory circuit consisting of the coil 5 and the condenser I4 controls a further high frequency stage of any particular kind, or the rectiiier, for example an audion. Between the grids G1 and G2 and the appertaining anodes A1 and A2 there are provided screening grids SG1, SGz, which in the known manner are linked up with a D. C. potential. Now accordingly to the subject matter of the present invention, in the rst place the capacity between G2 and K2 is to be smaller than that between G1 and K1. According to the additional subject matter of the invention, this is to be accomplished by the fact that the distance between G2 and K2 is made to be greater than between G1 and K1, or the fact that the surface or the diameter of K1 is made to be larger than that of K2, other conditions being the same.

I claim:

1. In an electronic amplier a tube having a ra and r1 being the radii of said other electrodes.

3. In an electronic amplifier a tube having an anode, a cathode, a screen grid placed close to said anode and a control grid, said control grid placed between said screening grid and said cathode at such a distance that the sum of the capacities between the screening grid and the control grid and between the control grid and the cathode is a minimum.

4. In an electronic ampliner a tube having an anode, a cathode, a screen grid placed close to said anode, a space charge grid placed close to said cathode and a control grid placed between said other two grids, said control grid placed at such a distance from each other that the sum of the capacities between the control grid and the screen grid and between the control grid and the space charge grid is a minimum.

5. In an electronic amplifier a tube having an anode, a cathode, a control grid placed close to the cathode and a screen grid, said screen grid placed between said anode and control grid at such a distance that the sum of the capacities between the screen grid and the control grid and between the screen grid and the cathode is a minimum. n

PAUL KAPTEYN. 

